Corrosion inhibitor



Patented Apr. 21, 1953 CORROSION INHIBITOR Gilson H. Rohrback, Whittier, Dwite M. McOloud, Buena Park, and Willard R. Scott, Alhambra, Calif., assignors to California Research Corporation, San Francisco, Calif., a corporation of Delaware No Drawing.

Application March 16, 1951,

Serial No. 216,080

7 Claims.

This invention relates to a corrosion inhibitor suitable for inhibiting the corrosion of ferrous metal tubing and piping in producing oil wells, more particularly this invention relates to a capsulate corrosion inhibitor comprising a metallic shell and an anti-corrosion filler.

The more efiective inorganic corrosion inhibitors such as the alkali metal dichromates and alkali metal phosphates and polyphosphates are water-soluble materials of relatively low density. It is difficult to place and maintain these materials at the bottom of a producing well, especially a well having a packed-off annulus which must be treated with a corrosion inhibitor by interrupting the production and placing the inhibitor at the bottom of the well through the tubing. When production is resumed these materials rapidly dissolve and disintegrate into small particles which are carried upward with the production stream. Their residence time and contact with the well tubing is therefore short and their application in this type of well is highly inefficient.

It has now been found that thes difiiculties may be overcome and that improvements in overall corrosion control may be obtained by the employment of the capsulate inhibitor cartridges of this invention.

In one embodiment of the invention, the corrosion inhibitor is a capsulate body comprising a metal shell composed of one or more metals having a standard oxidation-reduction potential above 0.5 and below 2.5 volts and a filler consisting of an inhibitor to acid attack on ferrous metals.

In a more specific and preferred embodiment of the invention, the corrosion inhibitor is a capsulate body comprising a metal shell composed of one or more metals having a standard oxidationreduction potential above 0.5 and below 2.5 volts and a filler consisting essentially of a semi-solid to solid material formed by intimately mixing arsenous oxide, water, and a material of the group consisting of alkali metal hydroxides, alkaline earth metal hydroxides and salts of alkali metals and alkaline earth metal with weak acids.

To prepare the capsulate corrosion inhibitors of this invention, metal tubing of suitable diamset in order that the metal casing of the capsulate inhibitor shall be anodic with respect to iron and shall be sacrificially corroded in the well. This corrosion accomplishes two useful purposes-first, the metal shell is rapidly corroded away in contact with the production stream so that the shell casings do not accumulate in the Well bottom as the use of the capsulate inhibitor is continued, and secondly, the metal shell is anodic with respect to the ferrous metal tubing of the well and being in contact with this tubing provides cathodic protection from electro-chemical corrosion. The upper level of the oxidation-reduction potential is set to exclude metals which are directly and rapidly attacked by water at ordinary temperature, such as sodium and calcium. Suitable metals are magnesium, aluminum, zinc, and alloys of these metals such as magnesium-aluminum alloys, magnesium-aluminum-zinc alloys and zinc-aluminum alloys, for example magnalium, the Dow metals, and ASTM alloy 23. The thickness of the metal shell is ordinarily in the range .01 to about .05 inch and more commonly in the range about .02 to about .04 inch. The shell must be thick enough to provide a durable container for the corrosion-inhibiting filler and should be thin enough to go into solution in the usual corrosion environment in a period less than about one week.

The diameter of the metal shell should be sufficiently small to permit ready passage of the capsulate inhibitor downward through the well tubing to the bottom of the well. Ordinarily the outside diameter of the shell is from one inch to one and one-half inches.

The shell length is usually from about one foot to four feet. Lengths in this range in combination with the diameter of the shell provide a casing capable of holding a charge of corrosion inhibitor sufficient to protect the metal tubin for a period of 24 hours or longer.

The shell as above described is filled with a corrosion inhibiting composition such as sodium dichromate, sodium polyphosphates, sodium hydroxide, sodium carbonate, sodium bicarbonate and the like. In a preferred embodiment of the invention, a magnesium shell is filled with a thick slurry of sodium hydroxide, water and arsenous oxide. After filling, the shell is allowed to stand for a period of about six hours during which the slurry sets to form a hard coherent solid mass.

After the shell is filled with the corrosion inhibiting composition, the open end is sealed. The sealing may be accomplished in several ways, for example the open end may be pinched together until only a narrow opening of the order of an eighth of an inch in width remains between the shell walls. Alternatively the open end may be sealed with a fairly high melting point wax, for example a wax'having a melting point of about 150F. The temperature at the well bottom and the oil contained in the production stream bring about a rapid removal of the wax capping by melting and dissolving it away. The open end of the casing may also be sealed'off with a water-soluble or oil-soluble: plastic material, for example a gelatin cap may be provided which is rapidly dissolved away by thewater contained; in the production stream leaving the upper endof the corrosion inhibitor filling of the casing in contact with the production stream.

The preferred capsulate corrosion inhibitorsf the invention having a magnesium shell and a filler consisting essentially of a thick slurry of arsenous oxide, sodium hydroxide and water were specifically prepared ;asfollows: Magnesium tubing having awall-thi'ckness of 0,035 inch and an outside diameterof 1. inchwas-cut intolengths of approximately two feet. Five partsby weight of sodium hydroxide andtwoparts by weight of waterwere stirredtogether. Tenlpa'rts by'weight of arsenousoxidewas slowlyaddedtothe aqueous sodium hydroxide: with. :gentle stirring. The arsenous oxide should be .addedzslowly since excessive heat is liberated by neutralization of the acid- -oxide with .the basic solution. After the arsenous oxide 1 has allibeen added-Itothe-:aqu'eous sodium-hydroxide, the mixture isstirred. to form 'asmooth thick paste. This-paste is then extruded into the magnesium tubes. Aiter' standing. for twenty-four hours without heatingthe pastehad set .to form a hard, solid-mass. Theopen ends of the-tube were then dipped in -a heavy wax havinga melting point of 150 Fi -to seal the tube.

Thick extrudable inhibitor pastes which set on standing are formed not only byemploying the specific-proportionsof water,alkali andarsenous oxide set out above,- but: by varying the '1 proportions ofthe ingredients within: the following ranges: to'3parts byweightof watc 2 110 10 parts by weight of "sodium hydroxide, and -to -2'O parts-by weight ofi arsenous oxide. The hardness and solution rate of t'he: solidified inhibitor paste may be varied by the addition ofsmall proportions of modifying materials such as' calci- 'umsulfate and-Portland cement if desired.

Corrosion .inhibitor cartridges preparedas above described were tested in a producing well. The well was 9,000 feet deep and produced 353 barrels of oil, 210 barrels of saltwater and 3g00O;- 000 cubic feet of gas daily; The gas con- 'tained-2% by weight. of carbon dioxide. The essentialxfeatures of the corrosive I environment of the well thus. included ferrous metal -in contact withoil, brine, and-gascontaining carbon dioxide, gas liquid interfaces-in contact with-the-metal, andareasof turbulent liquid flow in contact with themetal surfaces. The metal in this environmentis corrodedawayby directattack of carbonic (acid which is greatly accelerated by. electroschemicalphenomena .arising out of the contact of the phase interfaces with the metal and out .of. thecontactiof turbulent flowingsliquid with .the metal. surface. Substantially all: of the acid- :ity of the productionstream stems from the; presencevof. carbon. dioxide. The acidityis .quite low, the pH notbeing lower than 3. and being .ordi- .narily in the, range 4 to'6. .When: a ferrous metal ,is exposedto the-production: efiiuent under-quiesthe liquid or of free gas bubbles through the liquid, the corrosion rate observed is usually less than about 25% of that observed under actual producing conditions which include turbulent motion of the-liquid in contact with the metal and the presence of minute gas bubbles which contact the metal. In laboratory apparatus in which the separate influence of the three corrosive factors was studied, i. e., direct acid attack. electrochemical attack attributable to turbulent liquid flow, and electrochemical attack attributable to thepresence of finely-divided gas bubbles which contact the metal surfaces, it was found .that. therrelative rate of corrosion due to direct acid attack was 4 units, and that the relative rate of corrosion when either the condition of liquid turbulence or finely-divided gas bubbles moving through the liquid was superimposed on the presence of the acid medium, the relative corrosion rate immediately increased to 14 units. From these observations it is clear that thecorrosion problem would not be solved by the-employment of an inhibitor which would eliminate only the corrosive action due to direct acid attack and that the corrosion problem here presented can be solved only if the additional electrochemical corrosion attributable largelyto physical conditions ismarkedly reduced. The factis, if the only corrosion encountered in this corrosive environment were the corrosion attributable to direct acid attack, there would. be no corrosion problem and corrosion at the lowrate produced by this sourcealone could belaccepted since other operating factors would then becontrolling .in fixing equipment life and breakdown frequency.

Thecartridges usedin the test were somewhat larger than those above described,:being approximately three feet long, one and one-quarter inches in diameter, and containing three and'a half pounds of a sodium hydroxide-arsenous oxide-water paste. The cartridges were introduced into the well .by placing them above the master valve in the Christmas tree. Thehammer union at the bottom of the Christmas tree was closed and then the master valve was opened. The cartridges could be heard falling down the tubing at high velocity. In order to determine if the cartridges'were goingall theway down the well hole, they were followed'down with a wire line and parafiin scraper. The paraffin scraper reached the bottom-of the hole approximately thirty minutes after the cartridge'had been -.injected, showing that it had reached the bottom of the hole. A restricting device had been installed below the stringer-in-the tubing prior to the injection of the cartridgeso that: the cartridge would not fall beyond the tubing string.

The effectiveness of the corrosion inhibitor cartridges to inhibit the corrosion of ferrous metal tubing is determined by observing the iron count of the produced water before and during the treatment. The iron count, ameasure of the corrosion rate, is the number of parts per million of iron calculated as ferric oxide contained in the produced water as determined by the thiocyanate colorometric method (Scotts Standard Method of Chemical Analysis, 5th Ed., Van Nostrand, 1939, page 486). During the'test, one cartridge was introduced into the well each day during the first five days of the test-after which one cartridge wasintroduced on alternate days. During the test the iron count pick-up -of the production water was reduced from an average value of 19 prior to the commencement wcent-conditions where-there-ismo movement .of ;0f the test-to an average-value of.3 during the continuance of the test. This reduction represents the elimination of 84% of the corrosion normally sustained.

Of the several metals suitable for fabricating the shell of the capsulate corrosion inhibitor, magnesium is preferred. Magnesium is not amphoteric and is not attacked by alkaline slurries such as slurries of water, sodium hydroxide and arsenous oxide. Of the several corrosion inhibitors which may be employed as fillers for the shell, the arsenical compounds are preferred, especially a slurry of arsenous oxide, sodium hydroxide and a small amount of water as above described. These slurries, it should be noted, may also be prepared using sodium carbonate, sodium bicarbonate, potassium carbonate, sodium sulfide, potassium sulfite, calcium hydroxide, magnesium hydroxide, and the other alkali metal hydroxides and alkaline earth metal hydroxides or water-soluble salts formed from alkali metal hydroxides and alkaline earth metal hydroxides with weak acids instead of the sodium hydroxide exemplified above.

The capsulate corrosion inhibitors of this invention have several clear advantages over corrosion inhibitors heretofore employed by reason of the combination of the metallic shell and the water-soluble corrosion inhibitor constituting the filling. The capsulate corrosion inhibitors of this invention do not break during handling or injection. When the preferred arsenical inhibitors or other poisonous inhibitors constitute the filler for the capsule, the toxicity hazards incident to handling of such inhibitors are eliminated since personnel are protected from contact with the toxic compounds by the metal shell.

The metallic casing acts as a container for the dissolving corrosion inhibitor at the well bottom. Most flowing wells have open-end tubing and the stop screening must be run into the well to hold the corrosion-inhibiting cartridges. The strength of the container prevents particles of the filler from being chipped away from the mass by contact with the tubing walls or breaking away during dissolution to fall through the screen into the formation hole and out of the production stream.

The metal casings of the corrosion inhibitors of this invention provide a uniform and gradual solution of the inhibitor. The metal tubes remain intact in the well for about 24 hours, during which dissolution of the filler occurs only at the upper end of the tube which is exposed to the well water. This insures gradual solution of the inhibitor and gives longer opportunity for the inhibitor to contact the tubing surface to provide the desired protection from corrosion.

The metal shells of the capsulate inhibitors of this invention themselves provide protection against corrosion by two methods. The metals of this invention are themselves anodic with respect to iron in the present of a production stream comprising oil, water and carbon dioxide, and small quantities of organic acids. The metal shells are sacrifically corroded by electrochemical corrosion, thus preventing attack upon the iron. Further, the metals constituting the shell on going into solution provide a neutral to basic atmosphere in the immediate area of the arsenical filler, thus providing an atmosphere in which all of the arsenical corrosion inhibitor goes into solution in the well water. This is especially true of magnesium shells since magnesium reacts with water at well bottom temperatures 180 F. to form magnesium hydroxide and hydrogen.

We claim:

1. A capsulate corrosion inhibitor comprising a thin walled metal shell composed of at least one metal having a standard oxidation-reduction potential above 0.5 and below 2.5 volts filled with a semi-solid to solid material formed by intimately mixing sodium hydroxide, arsenous oxide and water.

2. An anti-corrosion cartridge comprising a thin walled metal shell composed of metals and metal alloys having a standard oxidation reduction potential above 0.5 and below 2.5 volts filled with an intimate mixture of arsenous oxide, water and a material of the group consisting of alkali metal hydroxides, alkaline earth metal hydroxides and salts of alkali metals and alkaline earth metals with Weak inorganic acids.

3. A corrosion-inhibiting cartridge suitable for introduction into producing oil wells, comprising a thin walled shell formed from metals and metallic alloys having a standard oxidation-reduction potential above 0.5 and below 2.5 volts filled with an intimate mixture of arsenous oxide, water and an alkali metal hydroxide.

4. A corrosion-inhibiting cartridge comprising a thin walled magnesium shell filled with an intimate mixture of to 3 parts by weight of water, 2 to 10 parts by weight of sodium hydroxide and 5 to 20 parts by weight of arsenous oxide.

5. A corrosion-inhibiting cartridge comprising a thin Walled metallic shell having a standard oxidation-reduction potential above 0.5 and below 2.5 volts filled with a slurry of arsenous oxide, sodium hydroxide and water.

6. The method of inhibiting corrosion of ferrous metal tubing in a producing oil well delivering a production stream comprising oil, water, and carbon dioxide gas through said tubing, which comprises periodically halting the production of the well, dropping a capsulate corrosion inhibitor comprised of a thin walled metallic shell having a standard oxidation-reduction potential above 0.5 and below 2.5 volts filled with a mixture of arsenous oxide, water and an alkali metal hydroxide through the tubing to the lower portion of the well, and producing the well.

7. A cartridge as defined in claim 3, wherein the shell is formed from magnesium.

GILSON H. ROHRBACK. DWITE M. MCCLOUD. WILLARD R. SCOTT.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,628,401 Haber May 10, 1927 1,638,710 Sherbins Aug. 9, 1927 1,877,504 Grebe et al. Sept. 13, 1932 2,031,632 Bottoms Feb. 25, 1936 2,385,175 Wachter Sept. 18, 1945 2,437,475 Oxford Mar. 9, 1948 2,546,586 Cross Mar. 27, 1951 

1. A CAPSULATE CORROSION INHIBITOR COMPRISING A THIN WALLED METAL SHELL COMPOSED OF AT LEAST ONE METAL HAVING A STANDARD OXIDATION-REDUCTION POTENTIAL ABOVE 0.5 AND BELOW 2.5 VOLTS FILLED WITH A SEMI-SOLID TO SOLID MATERIAL FORMED BY INTIMATELY MIXING SODIUM HYDROXIDE, ARSENOUS OXIDE AND WATER. 